Nested stent

ABSTRACT

An intraluminally implantable stent is formed of helically wound wire. The stent has a generally elongate tubular configuration and is radially expandable after implantation in a body vessel. The wire includes successively formed waves along the length of the wire. When helically wound into a tube, the waves are longitudinally nested along the longitudinal extent of the stent so as to form a densely compacted wire configuration. After radial expansion the stent maintains high radial compressive strength and wire density to retard tissue ingrowth.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of and claims priority to U.S.application Ser. No. 09/977,823, filed Oct. 15, 2001, now abandonedwhich is a continuation of U.S. application Ser. No. 09/271,304, filedMar. 17, 1999, now U.S. Pat. No. 6,319,277, which is a continuation ofU.S. application Ser. No. 08/708,651, filed Sep. 5, 1996, now U.S. Pat.No. 5,906,639, which is a continuation of U.S. application Ser. No.08/289,791, filed Aug. 12, 1994, now U.S. Pat. No. 5,575,816, which areall incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to implantable intraluminalstents and more particularly, the present invention relates to animproved high strength intraluminal stent having increased wire density.

BACKGROUND OF THE INVENTION

It is well known to employ endoprostheses for the treatment of diseasesof various body vessels. Intraluminal devices of this type are commonlyreferred to as stents. These devices are typically intraluminallyimplanted by use of a catheter into various body organs such as thevascular system, the bile tract and the urogenital tract. Many of thestents are radially compressible and expandable so that they may beeasily inserted through the lumen in a collapsed or unexpanded state.Some stent designs are generally flexible so they can be easilymaneuvered through the various body vessels for deployment. Once inposition, the stent may be deployed by allowing the stent to expand toits uncompressed state or by expanding the stent by use of a catheterballoon.

As stents are normally employed to hold open an otherwise blocked,constricted or occluded lumen; a stent must exhibit a relatively highdegree of radial or hoop strength in its expanded state. The need forsuch high strength stents is especially seen in stents used in theurogenital or bile tracts where disease or growth adjacent the lumen mayexert an external compressive force thereon which would tend to closethe lumen.

One particular form of stent currently being used is a wire stent.Stents of this type are formed by single or multiple strands of wirewhich may be formed into a shape such as a mesh coil, helix or the likewhich is flexible and readily expandable. The spaces between the coiledwire permit such flexibility and expansion. However, in certainsituations, such as when the stent is employed in the urogenital or biletract, it is also desirable to inhibit tissue ingrowth through thestent. Such ingrowth through the stent could have a tendency to recloseor occlude the open lumen. The open spaces between the wires forming thestent, while facilitating flexibility and expansion, have a tendency toallow such undesirable tissue ingrowth.

Attempts have been made to provide a stent which has less open space andmore solid wire. U.S. Pat. No. 5,133,732 shows a wire stent where thewire forming the stent is overlapped during formation to provide lessopen space. However such overlapping wire increases the diameter of thestent and has a tendency to reduce flexibility and make implantationmore difficult. It is therefore desirable to provide a wire stent whichexhibits high compressive strength and full flexibility without allowingextensive ingrowth therethrough.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an intraluminalstent which exhibits high compressive strength and is resistive totissue ingrowth.

It is a further object of the present invention to provide a flexiblewire stent having high compressive strength and maximum wire density toinhibit tissue ingrowth.

In the efficient attainment of these and other objects, the presentinvention provides an intraluminal stent including a generally elongatetubular body formed of a wound wire. The wire forming the stent isformed into successively shaped waves, the waves being helically woundalong the length of the tube. The longitudinal spacing between thehelical windings of the tube is formed to be less than twice theamplitude of the waves thereby resulting in a dense wire configuration.

As more particularly shown by way of the preferred embodiment herein, anintraluminal wire stent includes longitudinally adjacent waves beingnested along the length of the tubular body. The peaks or apices of thelongitudinally nested waves are linearly aligned. Further, theintraluminal stent so constructed would have a percentage of opensurface area in relationship to the total surface area of the stentwhich is less than 30% in the closed state, resulting in less open areaupon expansion which would inhibit tissue ingrowth.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a conventional helical coil formed of asingle wound wire.

FIG. 2 is a perspective view of the stent of the present invention.

FIG. 3 is a perspective view of the stent of FIG. 1 exhibitinglongitudinal flexibility.

FIG. 4 is a schematic showing of one wave of the wire forming the stentof FIG. 2.

FIG. 5 is a schematic showing of nested longitudinally adjacent waves ofthe stent of FIG. 2.

FIG. 6 is a perspective view of the stent of FIG. 2 shown in the open orexposed condition.

FIG. 7 shows a portion of a further embodiment of a wire used to form astent in accordance with the present invention.

FIG. 8 shows a still further embodiment of a wire used to form a stentof the present invention, partially wound around a forming mandrel.

FIG. 9 shows a membrane covering which may be employed in combinationwith the stent of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A simple helically formed coil spring 10 is shown in FIG. 1. Coil spring10 is formed of a single metallic wire 12 which for stent purposes maybe formed of a suitably flexible biocompatible metal. The wire coilspring 10 defines generally a cylindrical tubular shape which isradially expandable upon application of outward radial pressure from theinterior thereof.

The present invention shown in FIG. 2, improves upon the simple coilspring 10 shown in FIG. 1. However with reference to FIG. 1, certainterminology used hereinthroughout may be defined. As mentioned, thespring defines a generally elongate cylindrically tubular shape lyingalong a central axis χ. Wire 12 is helically wound, for example againsta constant diameter mandrel (not shown), to form a longitudinallyextending structure consisting of wire 12 and spaces or pitch 16therebetween. Each individual winding 14 may be defined as the wiresegment traversing one complete revolution around axis χ. As the wire ishelically coiled about axis χ, each winding is successivelylongitudinally spaced from the next adjacent winding by a givendistance.

For present purposes, the axial spacing between any point on the wirecoil spring 10 to the point defining the next successive winding may bethought of as the pitch 16 of the wire coil spring 10. As so defined,the pitch of the coil spring 10 defines the spacing between windings andtherefore the degree of compactness or compression of the wire coilspring 10.

Also with reference to FIG. 1, as the wire coil spring 10 has agenerally cylindrical tubular shape, it defines an outside diameter d₁and an inside diameter d₂ which would typically differ by twice thediameter d₃ of wire 12. Further, wire coil spring 10 generally definesan outer cylindrical surface area along its length which may be thoughtof as being composed of solid surface portions defined by the outwardfacing surface of wire 12 itself and open surface portions defined bythe spaces or pitch 16 multiplied by the number of wire windings 14. Theratio of open surface space to solid surface space may be varied byvarying the so-defined pitch 16 of the wire coil spring 10. A smallerpitch coil, where the windings are more compacted or compressed, wouldresult in an outer surface area having less open space than a coilformed to have greater spacing or pitch between the wire windings.

Having set forth the definitional convention used hereinthroughout, thepresent invention may be described with reference specifically to FIGS.2-6. A wire stent 20 of the present invention is shown in FIG. 2. Wirestent 20 is generally in the form of an elongate cylindrically shapedtubular member defining a central open passage 21 therethrough. Stent 20is formed of multiple windings 24 of a single wire 22 which in thepresent invention is metallic, preferably tantalum, as such wireexhibits sufficient spring elasticity for purposes which will bedescribed in further detail hereinbelow.

While stent 20 may be formed by helically winding wire 22 much in amanner shown with respect to FIG. 1 to form wire coil spring 10, thepresent invention contemplates preshaping the wire 22 itself along itslength prior to helically coiling the wire.

Referring now to FIG. 4, wire 22 in an elongate pre-helically coiledconfiguration may be shaped in a manner having a longitudinallyextending wave-like pattern. Wave pattern 25 is defined by a pluralityof continuously repeating wave lengths 27 therealong. It has been foundadvantageously that the waves may take the form specifically shown inFIGS. 4 and 5 for optimum results as a wire stent. However, forexplanation purposes, the wave-like pattern 25 generally functionsmathematically as sinusoidal wave, having a given amplitude A asmeasured from a central axis y and a peak-to-peak amplitude of 2 A. Thewave pattern 25 has a uniform preselected period λ equal to thetransverse extent of a single wave length. The geometry of each wavelength 27 is shown in FIG. 4.

The wave-like configuration imparted to wire 22 may be accomplished in avariety of forming techniques. One such technique is to pass wire 22between the teeth of intermeshed gears (not shown) which would place agenerally uniform sinusoidal wave-like crimp along the length of thewire. Other techniques may be used to form the specific shape shown inFIG. 4. Wire 22 may be passed through a pair of gear-like overlappingwheels (not shown) having depending interdigitating pins. By arrangingthe size, position and spacing of the pins, various wave-likeconfigurations may be achieved. The particular shape shown withreference to FIGS. 4 and 5 has been selected as each wave length 27includes a pair of non-curved linear sections 29 between curved peaks31. As will be described with respect to FIG. 5, this configurationallows the waves to be stacked or nested with maximum compactness whenthe wire is helically wound around a forming mandrel (FIG. 8) into theshape shown in FIG. 2.

Referring now to FIG. 5, schematically shown is a portion of stent 20 ofFIG. 2 which has been cut once, parallel to the χ axis and flattenedafter being wound in a helical fashion such as that described withrespect to the wire coil spring 10 of FIG. 1. Wire 22 formed in themanner shown and described with respect to FIG. 4, may be helicallywound around an appropriately shaped mandrel (FIG. 8). The width of themandrel is selected in combination with the frequency and period of thewaves forming wire 22 so that upon helical coiling therearound the wavesforming each winding 24 are longitudinally stacked or nested within thewaves formed by the longitudinally adjacent winding successively spacedtherefrom.

As can be seen with respect to FIG. 5, the peaks 31 of the waves oflongitudinally adjacent windings 24 are each linearly aligned so thateach wave is stacked or nested within the next adjacent wave. In optimumconfiguration, the spacing or pitch 26 between each longitudinallysuccessive winding 24 is constructed to be minimal. However, nesting orstacking does occur where the pitch or spacing between longitudinallyadjacent windings 24 is less than 2 A i.e. the peak-to-peak amplitude.As long as the pitch remains less than 2 A each longitudinally adjacentwinding 24 will be nest d within the wave formed by the previouslyformed winding 24. By minimizing the pitch or spacing 26 betweenadjacent windings 24, the open space between windings may be minimized.The particular wave-like pattern imparted to wire 22 as shown in FIG. 4allows particularly tight stacking of longitudinally adjacent windings.

The particular configuration of the stent 20 shown in FIG. 2, providessignificant advantages in medical applications. The stent 20 of thepresent invention is typically implanted by means of a balloon catheter(not shown). The stent 20 in a closed form is held around a deflatablecatheter balloon. The stent is then inserted into the lumen and locatedat the desired position. The shape of the closed stent shown in FIG. 2permits ease of insertability. As shown in FIG. 3, the stent may beeasily bent or flexed along its longitudinal extent. The spacing orpitch 26 of windings 24 facilitate such bending. This helps in theinsertion and deployment of the stent through a lumen, as typically bodylumens traverse a torturous path through the body which must be followedby the stent which is being deployed therein. Once properly located, theballoon is inflated and the stent is radially expanded for deployment.The balloon is then deflated, and the catheter is removed leaving theexpanded stent in place.

The windings of stent 20 in closed condition are tightly nested. Thecylindrical surface area formed by the coiled wire has greater wiredensity, i.e. more of the surface area is composed of solid wire whileless of the surface area is composed by open space between the wirewindings then in previous non-nested single wire stents. The wiresurface area in the closed condition equals the wire surface area in anexpanded condition. By maximizing the closed condition wire surfacearea, even when the stent is expanded such as shown in FIG. 6, theexpanded wire surface area is also maximized reducing tissue ingrowthbetween the expanded windings of the stent. Contrary to a simple coilspring such as that shown in FIG. 1, the stent 20 of the presentinvention expands without significant foreshortening of the stent orrotation of the ends of the coil. Rather, expansion is achieved by aflattening or elongation of the individual waves of the stent 20. Oncethe stent is expanded after deployment to a shape shown in FIG. 6, theincreased wire surface area as well as the particular shape of the wireprovides sufficient radial strength to resist the compressive forces ofa blocked, constricted or impinged upon lumen.

Additionally, the above-described benefits of the stent of the presentinvention are achieved without the necessity of longitudinallyoverlapping adjacent wire windings. In many prior art stents, the stentsinclude portions of wire windings which are longitudinally overlapped.This increases the wall thickness of the stent thereat and results in astent which is more difficult to implant in the body lumen by means of aballoon catheter. Also, such stents create an undesirable, moreturbulent fluid flow therethrough. The stent of the present inventionmaximizes wire density, maintains a high degree of flexibility andradial compressive strength without increasing the stent wall thicknessbeyond the single wire diameter.

EXAMPLE

Mathematically, the geometric analysis of the preferred embodiment ofthe stent of the present invention may be described as follows withreference to FIGS. 4 and 5.

Each wave length 27 of the wave pattern 25 forming stent 20 is formed toinclude a straight leg segment 29 with a bend radius at peak 31. Theangle at which the helix coils around the center line χ (FIG. 1) isassumed to be close to 90°, so that the successive windings 24 arepositioned to b as close to concentric as possible while stillmaintaining a helical pattern.

The integer number of waves N per single circumference or single windingfollows the equation:

${N = \frac{\pi\; D}{\lambda}};$

where D is the diameter of the closed stent and λ is the period of asingle wave.

The number of helical windings M per stent is defined by the equation:

${M = \frac{L\;\sin\;\theta}{d_{3}}};$

where L is the overall stent length; θ is the angle of the straight legsegments 29 with respect the line of amplitude of the wave pattern; andd₃ is the wire diameter.

The exterior exposed surface area of the stent is equivalent to theamount of wire packed within a fixed stent length. The total lengthL_(w) of wire employed to form the stent follows the equation:

$L_{w} = {M\;{N\left( {{4\; l} + {4\left( {r + \frac{d_{3}}{2}} \right)\frac{\pi}{180}\left( {90 - \theta} \right)}} \right)}}$

where r is the radius defining the peak curvative; and l is the lengthof the straight line segment 29 of the wire.

It follows that the projected solid wire area is L_(w)d₃ and thepercentage of open space coverage (% open) is given by the equation:

${\%\mspace{14mu}{OPEN}} = {100\left( {1 - \frac{L_{w}d_{3}}{\pi\mspace{14mu}{DL}}} \right)}$

In a specific example, a stent having the parameters listed in Table Iand formed in accordance with the present invention yields a percentageof open space (% open) equivalent to 28.959%.

TABLE I L Length of Stent 1.000 in D Diameter of Closed Stent 0.157 ind₃ Wire Diameter 0.010 in r Radius of Curvative of Peak 0.020 in NNumber of Waves per Winding 3 M Number of Windings per Stent 22.47 lLength of Straight Portion of Stent 0.097 in

Further, it is found that an expanded stent constructed in accordancewith the example set forth above, exhibits superior resistance topressure P acting upon the stent in a radially compressive manner (FIG.6). In the present and illustrative example, P has been has beendetermined, both mathematically and empirically, to be 10 psi.

It is further contemplated that the stent of the present invention maybe modified in various known manners to provide for increased strengthand support. For example the end of wire 22 may be looped around anadjacent wave or extended to run along the length of the stent. The wiremay be welded to each winding to add structural support such as is shownin U.S. Pat. No. 5,133,732. Also, each windings may be directly weldedto the adjacent winding to form a support spine such as shown in U.S.Pat. No. 5,019,090.

Further, as mentioned above, wire 22 is helically wound around a mandrelto form the helical pattern shown in FIG. 1. While the angle at whichthe helix coils around the mandrel is quite small, a certain angle mustbe imparted to the uniform windings to form a coil. It is furthercontemplated that a helix-like winding may be formed by concentricallywrapping a wave pattern around the mandrel where the length of the sidesof each wave are unequal. As shown in FIG. 7 a wave pattern 125 may beformed having leg segments 129 of uneven length. Wave pattern 125includes individual wave lengths 127 having a first leg segment 129 aand a second leg segment 129 b. Leg segment 129 a is constructed to beshorter than leg segment 129 b. Thus wave pattern 125 has a step-typeshape so that upon winding around a mandrel, the windings 124 coil in ahelical-like fashion therearound. This provides a lengthwise extent tothe coil without having to impart a helical wrap thereto. Forming thestent length in this manner may tend to result in better flowcharacteristics through the stent in use.

Other modifications which are within the contemplation of the presentinvention may be further described. FIG. 8 shows a wire 222 which hasbeen preformed to have a wave pattern 225 which is generally triangularin shape. This wave pattern 225 includes individual wave lengths 227having straight leg segments 229 a and 229 b which meet at an apex 231.Wire 222 so formed, may be wound around a mandrel 200. As the individualwave lengths 227 nest in a manner above described, the apices 231 of thewave length 227 are longitudinally aligned.

The winding of wire 222 around mandrel 200 takes place in the followingmanner. The formed wire 222 is held in position while the mandrel isrotated in the direction of arrow A, thereby coiling the wire 222 aroundmandrel 200. The spacing or pitch 216 is created by subsequent verticalmovement of the of the formed wire 222 along mandrel 200 while rotationthereof is taking place. When the winding is complete, the ends 233 ofthe wire 222 may be “tied off” by looping the end 233 around the nextlongitudinally adjacent winding.

While in the embodiment shown above, the amplitude of each wave isrelatively uniform, it is contemplated that the wire could be formed tohave waves of varied amplitude. For example, the wire could be formed sothat at the ends of the wound stent the amplitude of the waves isrelatively small while in the central portion of the stent the amplitudeis relatively large. This provides a stent with a more flexible centralsection and more crush-resistant ends.

In certain situations the stent of the present invention may include amembrane covering (not shown) which would cover the entire stent. Thewire surface of the stent would serve as a support surface for themembrane covering. The membrane covering would act as a further barrierto tissue ingrowth. Any membrane covering may be employed with thepresent invention such as a fabric or elastic film. Further, thismembrane covering may be completely solid or may be porous. In addition,as above described, employing a formed wire having varied amplitudewhere the amplitude of the wire is smaller at the ends of the stentwould help support the membrane covering as the crush-resistant endswould serve as anchors to support the membrane covering with littlesupport necessary at the more flexible central section of the stent.

Various changes to the foregoing described and shown structures wouldnot be evident to those skilled in the art. Accordingly, theparticularly disclosed scope of the invention is set forth in thefollowing claims.

1. An intraluminal device for implantation into a body lumen comprising:an elongate tubular stent formed of a helically wound wire defining aplurality of wire waves wherein said wire waves are longitudinallynested within each other; the stent having an unexpanded state, theplurality of wire waves comprising a first wire wave and a second wirewave, the first wire wave being longitudinally adjacent to the secondwire wave, wherein at least a portion of the first wire wave is indirect contact with at least a portion of the second wire wave in theunexpanded state; the stent having a length; and a lumen defined by acovering extending along at least a portion of the length of the stent,wherein the stent defines an open area between portions of thelongitudinally adjacent wire waves, the percentage of open area inrelation to the total surface area of the stent is less than 28%, whenthe stent is in the unexpanded state.
 2. An intraluminal device of claim1 wherein said waves are defined by a given amplitude and wherein saidgiven amplitude of the waves varies along the length of said stent. 3.An intraluminal device of claim 2 wherein said amplitude of the wavesadjacent the ends of the stent is smaller than the amplitude of thewaves therebetween.
 4. An intraluminal device of claim 1 wherein saidcovering is porous.
 5. An intraluminal device of claim 1 wherein saidcovering is solid.
 6. An intraluminal device of claim 1 wherein saidcovering is elastic.
 7. An intraluminal device of claim 6 wherein saidcovering is supported continuously along said tubular body.
 8. Anintraluminal device of claim 1 wherein said covering is formed from amembrane.
 9. An intraluminal device of claim 1 wherein covering isgenerally cylindrical.
 10. An intraluminal device of claim 1 whereinsaid covering is formed of a film.
 11. An intraluminal device of claim10 wherein said film is porous.
 12. An intraluminal device forimplantation into a body lumen comprising: an elongate tubular stentdefined by a plurality of helically wound wire waves, each wire wavedefined by an amplitude; longitudinally adjacent wire waves having apeak-to-peak distance, wherein the peak-to-peak distance is less thantwo times of the amplitude of at least one of the longitudinallyadjacent wire waves; the stent having an unexpanded state, the pluralityof wire waves comprising a first wire wave and a second wire wave, thefirst wire wave being longitudinally adjacent to the second wire wave,wherein at least a portion of the first wire wave is in direct contactwith at least a portion of the second wire wave in the unexpanded state;the stent having a length; and a covering extending along at least aportion of the length of the stent, wherein the stent defines an openarea between portions of the longitudinally adjacent wire waves, thepercentage of open area in relation to the total surface area of thestent is less than 28%, when the stent is in the unexpanded state.